High-refractivity low-dispersion glasses are highly demanded as materials for optical elements such as various lenses, and the like. As a glass having such optical constants, for example, there are known a dense tantalum flint glass TaSF17, etc., as disclosed on page 106 of “Glass Composition Handbook” compiled by Hiroshi Ogawa and Shinei Ogawa and issued by Japan Glass Product Industry Society (1991).
In recent years, with rapid popularization of digital cameras, video cameras, etc., demands for glass lenses as parts are increasing. On the other hand, with an increase in the number of pixels of imaging device in digital cameras, optical elements are required to have high performances, and there are demanded the ready supply of optical elements having high form accuracy.
A precision press-molding method is known as a method for highly productively producing optical elements formed of a glass having high form accuracy for supplying them. Since, however, all of conventional glasses such as TaSF17 and NbSF32 have high glass transition temperatures, they are not suitable for the precision press-molding.